1. Field of the Invention
This invention relates to the protection and pretreatment of aluminum and aluminum alloys and to the surface treatment of aluminums with novel compositions and to the method of using these compositions for the pretreatment of aluminum and aluminum alloys at ambient temperatures. The pretreatment coatings provide improved corrosion resistance, adhesion of overlaying coatings e.g. paints etc., and maintains low electrical contact resistance in a corrosive environment. More specifically, this invention relates to a corrosion resistant composition and to the process of using the composition on aluminum and aluminum alloys which comprises an acidic aqueous solution containing effective amounts of at least one water soluble trivalent chromium salt, an alkali metal hexafluorozirconate, at least one fluoro-compound and at least one water soluble thickener and/or water soluble surfactant.
2. Description of Prior Art
It is the current practice to improve the corrosion resistance and bonding of subsequent coatings to aluminum and its alloys by initially coating or pretreating the metal surface with protective films. The coating enhances the corrosion resistance of the unpainted metal surface and prepares the surface for a finish coating such as paint etc. These conversion coatings are most often applied by the use of hexavalent chromium-containing solutions. While these coatings provide good corrosion resistance, attempts have been made to provide a more acceptable non-chromate derived coating because of the growing concern regarding the occupational safety, health and environmental effects of hexavalent chromium. Hexavalent chromium is highly toxic and is a known carcinogen. Therefore, the solutions used to deposit these protective films and the films per se are toxic carcinogenic. Chromate films, however, provide outstanding paint adhesion and corrosion resistance and are easy to apply by various methods including immersion, spraying or by the wipe-on technique. However, the environmental laws and OSH regulations are forcing the military and commercial users to find other non-toxic, non-chromate pretreatments. Moreover, the use of chromate conversion coatings is becoming more expensive as the regulations are being enforced and costs become prohibitive with the restrictions being imposed by the EPA In addition, certain processes like spraying chromate conversion coatings are forbidden because of OSH, thereby forcing the use of less than optimum alternative methods.
More specifically, it is known that aqueous chromate solutions contain chemicals that partially dissolve the surface of the metal and form insoluble films known as chromate conversion coatings or pretreatments. These coatings are corrosion resistant and protect the metal from various elements which cause corrosion. Although the conversion coatings enhance corrosion resistance and improve the paint bonding properties, the coatings have a serious drawback, i.e., the toxic nature of the solutions from which they are made and the presence of hexavalent chromium in the applied films. This is a serious problem from the standpoint of the operators handling the solution e.g. disposing the used chromate solution, the chromate-contaminated rinse water, and the coating systems contaminated with chromates. These problems, however, can be avoided by eliminating the hexavalent chromium from the process. However, this method is expensive and can be a major cost factor in the overall metal treating process. Therefore it is highly desirable to provide processes and protective coatings which are free of hexavalent chromium, but at the same time capable of imparting corrosion resistant and bonding properties which are comparable to those imparted by conventional chromate-based conversion coatings. Of particular interest is the use of chromate conversion coatings on aluminum and its alloys e.g. the coating of large objects such as aircraft. It would be desirable to provide a protective coating for aluminum and its alloys utilizing relatively non-toxic chemicals that could serve as an alternative to the toxic hexavalent chromium.
This invention relates to acidic aqueous trivalent chromium pretreatment (TCP) of aluminum and its alloys and to the process for improving the adhesion, and corrosion resistant properties of the aluminum substrates. Specifically, this invention relates to pretreating aluminum and to the process of pretreating aluminum substrates at ambient temperatures or higher e.g. temperatures ranging up to about 200xc2x0 F. The pretreatment solutions comprises an acidic aqueous solution having a pH ranging from about 2.5 to 4.5 and preferably from 3.7 to 4.0, and contains per liter of said solution, from about 0.01 to 22 grams of a water-soluble trivalent chromium salt, about 0.01 to 12 grams of an alkali metal hexafluorozirconate, about 0.01 to 12 grams of at least one fluoro-compound selected from the group consisting of an alkali metal tetrafluorobroate, an alkali metal hexafluorosilicate and various combinations thereof, from about 0 to 10 grams per liter and preferably 0.5 to 1.5 grams of at least one water-soluble thickener, and from 0 to 10 grams per liter and preferably 0.5 to 1.5 grams of at least one water-soluble non-ionic, cationic or anionic surfactant.
It is therefore an object of this invention to provide pretreated aluminum and its alloys by treating said aluminums with an aqueous solution comprising a trivalent chromium salt, an alkali metal hexafluorozirconate, and effective amounts of a tetrafluoroborate and/or hexafluorosilicate to improve the electrical resistance, adhesion and corrosion resistance properties.
It is another object of this invention to provide a process of pretreating aluminum substrates with a stable acidic aqueous solution having a pH ranging from about 3.7 to 4.0 containing effective amounts of a trivalent chromium salt and hexafluorozirconate.
It is a further object of this invention to provide a stable acidic aqueous solution having good xe2x80x9cshelf lifexe2x80x9d containing trivalent chromium salts, hexafluorozirconate, and a pH ranging from about 3.7 to 4.0 for pretreating aluminum substrates at ambient temperatures wherein said acidic pretreating solution contains substantially no hexavalent chromium.